Positive and Negative Control of Gene Expression
- Gene Regulation is brought about by Regulatory Proteins.
- Regulatory proteins (activators and repressors) encoded by the regulatory genes that control the gene expression
What are regulator genes and structural genes?
- A structural gene is simply any gene that codes for a protein (or RNA) product. Structural genes represent an enormous variety of protein structures and functions including structural proteins, enzymes with catalytic activities, and regulatory proteins.
- A regulator gene simply describes a gene that codes for a protein (or RNA) involved in regulating the expression of other genes.
There are two basic types of transcriptional control: negative regulation and positive regulation.
In negative control: Regulatory protein acts as a repressor, inhibiting transcription
When a regulatory protein (repressor) binds to the operator sequences in DNA, the ongoing transcription is inhibited or repressed.
In positive control: regulatory protein acts as an activator, stimulating transcription.
When a regulatory protein (activator) binds to activator binding sequences in DNA, it stimulates already initiated low level of transcription of structural genes.
Regulatory proteins may be assisted by coregulators
Coregulators are proteins that do not bind to DNA by themselves, but that facilitate communication between sequence-specific regulatory proteins and the basal RNA polymerase machinery.
Two coregulators work to control repressors: inducers and corepressors (Table 1).
1) Inducer – Inactivates an active repressor (stimulating the expression of an operon)
How they act: Inducer will bind to the repressor and induce a conformational change in the repressor. As result repressor not able to bind to the operator.
2) Corepressor – Activates an inactive repressor
How they act: It will bind to an inactive repressor (aporepressor) and activates it. The repressor–corepressor complex then binds to the operator, preventing expression of the operon.
Two coregulators work to control activators: inhibitors and coactivators (table 1).
1) Inhibitor: Inactivates an activator
How they act: Inhibitor prevents the binding of activator to an activator binding sequence thereby repressing the operon.
2) Coactivator – Activates an inactive activator
How they act: It will bind to an inactive activator (apoactivator) resulting in an active activator that stimulate transcription of the operon.
Regulatory proteins often control their own synthesis: Autoregulation
- Regulatory proteins are to be maintained frequently at a critical level in the cell.
- This is achieved by autoregulation which can involve positive (auto-activation) or negative (auto-repression) control.
- Higher cellular concentrations of the regulatory protein tend to make it repress its own promoter limiting its transcription whereas low cellular concentrations of the regulatory protein make it activate transcription from its own promoter.